((O Muhammad): What thing is of most weight in testimony?) [6:19].
Said al-Kalbi: “The chiefs of Mecca said: ‘O Muhammad, we see that no one believes in what you say concerning the matter of messengership. We have asked the Jews and Christians about you and they claim that there is no description or mention of you in their Scriptures. So show us someone who will testify that you are a messenger, as you claim’. As a response, Allah, exalted is He, revealed this verse”.
Asbab Al-Nuzul by Al-Wahidi , trans. Mokrane Guezzou